1. Field of the Invention:
This invention relates to storage-stable anaerobic curing adhesive compositions containing polymerizable acrylate ester monomers and to an improved process for the preparation thereof. The compositions are stable for periods of time extending to a half-year or more in the presence of oxygen, but polymerize (cure) rapidly in the absence of air or oxygen, thereby finding use as anaerobic adhesives and sealants.
2. Description of the Prior Art:
Anaerobic curing compositions are well known and described in the prior art. U.S. Pat. No. 2,628,178 describes the preparation of anaerobic adhesives which rely on the oxygenation of certain polyacrylate monomers until at least 0.1% active oxygen is introduced into the monomer. The resultant oxygenated adhesive composition is then stored in contact with a continuous current of air or oxygen until polymerization is desired (at which time polymerization is initiated by the absence of air). This process therefore has the disadvantage that it is totally unsuited to the preparation and distribution of small quantities of adhesive sealants, since it is clearly impracticable to maintain a constant supply of air or oxygen to a large quantity of containers over an extended period of commercial conditions.
U.S. Pat. No. 3,795,641 to Lees et al. teaches that such constant oxygen supply during storage need not be employed when the monomer is initially oxygenated in the presence of minor amounts of an organic tertiary amine and/or an imido compound. This process, however, suffers from several limitations and the subject invention is considered to be an improvement thereon. Thus, aside from economic considerations, there is a measure of inconvenience in supplying and employing the required oxygen-bearing gaseous stream. Moreover, the oxygenation period itself is relatively lengthy, being measured in days in many instances. Additionally, the resulting adhesive is not characterized by a superior degree of stability and cure speed desired in many commercial products.
Other methods have been developed for rendering monomers to possess anaerobic properties without the necessity for oxygenating the monomers. These methods require the presence of specific polymerization initiators such as organic hydroperoxides, peroxy or perester compounds, sulfones, diazonium salts and the like, as disclosed, for example, in U.S. Pat. Nos. 2,895,950; 3,775,385; 3,957,561 and 3,054,480. Additional patents directed to improvements on the above-mentioned catalyzed systems include, for example, U.S. Pat. No. 4,038,475, which teaches the use of a sulfimide and amine as an accelerator system for the peroxy-initiated polymerization of an unoxygenated monomer, the adhesive being stabilized with a selected quinone and containing a chelating agent.
It is an object of the present invention to provide an anaerobic curing adhesive composition which does not require the use of externally added polymerization initiators.
It is another object to provide an anaerobic curing adhesive composition which does not require constant passage of gaseous oxygen either during preparation of the adhesive or during storage thereof.
It is an additional object of the invention to provide an anaerobic curing adhesive composition which is characterized by superior stability and cure speed.